The invention relates to brushless D.C. motor control, and in particular to driving a brushless D.C. motor.
A typical brushless D.C. motor includes a plurality of permanent magnets attached to a rotor, one or more pole pairs and a stator having a plurality of phases. The motor can be actuated by generating a rotating magnetic field that varies, with respect to time, current flow into the stator windings, particularly, e.g., a block commutation of the phases, with the help of control electronics. The rotor follows the rotating magnetic field, thereby rotating the rotor.
Timing the rotating magnetic field, and in particular directing the current flow through the stator as a function of rotor position, is crucial for proper operation of the motor. For example, where the stator field is ahead of the rotor, the rotor may no longer be able to follow the field and, therefore, be blocked. Where the rotor is ahead of the stator field, it may be braked and, therefore, the motor may no longer be able to develop its full torque.
A sensor, such as a Hall sensor, can be used to provide information on the actual position of the rotor to facilitate proper timing. However, disadvantageously, such an additional sensor adds to the costs associated with the motor.
Alternatively, voltage that is back-induced (i.e., a back-electro-motive force, BEMF) from the rotating rotor into the stator winding can be observed, particularly on a non-energized stator winding, to provide information on the actual position of the rotor. This positional information may be obtained by detecting the zero passages of the BEMF. Examples of such methods are disclosed in U.S. Publication Nos. US 2007/0013330 and U.S. 2006/0170383, and in German Publication Nos. DE 69831776 and DE 69017152.
The BEMF may be evaluated in a computer-supported manner using a rapid A/D converter. Results from the A/D converter are compared, using software, to a reference value corresponding to the zero passage of the BEMF. Disadvantageously, however, this method requires providing the rapid A/D converter, a powerful CPU to run the zero passage detection software, and also an additional reference voltage source, an external reference input at a neutral point of the motor or a virtual neutral point to provide the reference value. In addition, where the motor is regulated via pulse-width modulation (PWM), the A/D converter and the motor voltage must be synchronized, or a filtering circuit that prolongs signal propagation times must be provided.
Alternatively, an additional circuit may be configured with the control circuit that (i) compares the BEMF to a reference voltage corresponding to the zero passage, and (ii) supplies a trigger signal to a microcontroller when the reference voltage is either exceeded or is not reached. Disadvantageously, however, this additional circuit increases both manufacturing costs and circuit complexity.
There is a need for an improved technique for driving a brushless D.C. motor after starting up.